In comparison with a vibrated concrete (VC) of the same strength class, self-compacting concrete (SCC) typically has a lower coarse aggregate content and, possibly, a smaller maximum aggregate size. This may result in reduced aggregate interlock between the fracture surfaces of a SCC. Since aggregate interlock plays an important role in the shear strength of slender beams, SCC beams may have a shear strength lower than that of similar VC beams, but studies on that subject are still limited.
This article summarizes an experimental programme that includes beams of high-strength SCC and transverse reinforcement ratios around the minimum given by different codes - a case that had not been investigated so far. The shear strengths of those SCC beams are compared with those of VC beams with similar concrete compressive strength and small ratios of transverse reinforcement and also compared with beams calculated according to different code procedures.

Die aktuellen Entwicklungen in der Windenergiebranche führen zu Windenergieanlagen mit immer höherer Leistungsfähigkeit. Als Konsequenz aus dieser Entwicklung steigen mit der Leistungsfähigkeit der Anlagen auch die Beanspruchungen der Konstruktion. Hochleistungsbetone mit selbstverdichtenden Eigenschaften werden in Windenergieanlagen schon seit einiger Zeit zur Herstellung von Verbindungen zwischen den einzelnen Bauteilen verwendet. Darüber hinaus werden derzeit aufgelöste Gründungskonstruktionen und Türme aus Spannbeton, für die hochfester Beton eingesetzt wird, entwickelt. Diese Hochleistungsbetone sind für den Grenzzustand der Ermüdung zu bemessen, was zukünftig nach fib-Model Code 2010 erfolgen wird. Dieser enthält ein geändertes Ermüdungsbemessungsmodell für Beton, das auch für hochfeste Betone zu sicheren und wirtschaftlichen Bemessungsergebnissen führt. Am Institut für Baustoffe der Leibniz Universität Hannover wird seit Jahren das Ermüdungsverhalten von Hochleistungsbetonen untersucht. Darüber hinaus wird auch der Einfluss von Stahlfasern auf das Ermüdungsverhalten von Hochleistungsbetonen erforscht. In diesem Beitrag werden aktuelle Forschungsergebnisse zum Ermüdungsverhalten verschiedener Hochleistungsbetone mit und ohne Stahlfaserverstärkung vorgestellt und darauf aufbauend der praktische Einsatz dieser Betone diskutiert. Die Ergebnisse der durchgeführten Untersuchungen werden im Kontext zum neuen Ermüdungsbemessungsmodell des fib-Model Code 2010 interpretiert.

Fatigue behaviour of high performance concretes for wind turbines
New developments in the wind energy sector will lead to wind turbines with enormous capacities. As a result, the loads of the supporting structures are also increasing. For some time now, high performance concretes with self-compacting properties have been used in wind turbines for structural connections. Furthermore, slender foundations and prestressed concrete supporting structures made out of high-strength concrete are under development. In future, fatigue design of these high performance concretes is to be done according to the new fib-Model Code 2010. This code includes a new fatigue design model which enables a safe and economic fatigue design, even for high strength concrete. Extensive research with regard to the fatigue behaviour of different types of high performance concrete has been carried out at the Institute of Building Materials Science, Leibniz Universität Hannover. As part of these research activities, the influences of steel fibre reinforcement on the fatigue behaviour of high performance concretes are being investigated. In this paper, interim results of these investigations are presented and the potential for the practical applications of high performance concrete is discussed. The results of the conducted investigations are presented in comparison with the new fatigue design model of the fib-Model Code 2010.

Fatigue design according to CEB-FIP Model Code 90 is limited to concrete grades up to C80. In addition, the design rules include a strength-dependent reduction in the fatigue reference strength, which leads to uneconomical design of high-strength concrete. Considering comprehensive knowledge now available concerning the fatigue behaviour of normal-strength and high-strength concretes, the amount of this reduction can no longer be justified. A new design model for compressive fatigue loading and its derivation is presented in this article. A comparison between the new design model and the current standard ones reveals that the new design model ensures safe and economical design of normal-strength, high-strength and ultra-high-strength concrete. This new design model is included in the new fib Model Code 2010.

Der Ermüdungsnachweis nach CEB-FIP Model Code 90 ist auf Betone bis zu der Festigkeitsklasse C120 beschränkt. Gleichzeitig beinhaltet dieser Nachweis eine festigkeitsabhängige Reduktion der Druckfestigkeit unter Ermüdungsbeanspruchung, die bereits für hochfeste Betone zu einer unwirtschaftlichen Bemessung führt. In Anbetracht der mittlerweile vorliegenden umfangreichen Erkenntnisse zum Ermüdungsverhalten normalfester und hochfester Betone ist die Größe dieser Abminderung als nicht mehr gerechtfertigt einzustufen. Im vorliegenden Beitrag wird ein neu entwickeltes Ermüdungsbemessungsmodell vorgestellt, welches eine wirtschaftliche und sichere Bemessung von normalfesten, hochfesten und ultrahochfesten Betonen im Grenzzustand der Ermüdung ermöglicht. Der neue Bemessungsansatz wurde in das Bemessungskonzept des CEB-FIP Model Code 90 integriert, so dass eine einfache Anwendung möglich ist.

Design model for the fatigue behaviour of normal strength, high strength and ultra-high strength concrete
Fatigue design according to CEB-FIP Model Code 90 is limited to concrete grades of up to C120. In addition, the design rules include a strength dependent reduction of the fatigue reference strength, which leads to uneconomical design of high strength concretes. Considering today's existing comprehensive knowledge regarding the fatigue behaviour of normal strength and high strength concretes, the size of this reduction is no more justifiable. In this article, a newly developed design model for fatigue is presented, which enables an economic and safe design of normal strength, high strength and ultra-high strength concretes based on fatigue limit state. This new design approach is integrated into the design concept of the CEB-FIP Model Code 90, thereby guaranteeing a simpler application.

Conservation of concrete structures forms an essential part of the fib Model Code for Concrete Structures 2010 (fib Model Code 2010). In particular, Chapter 9 of fib Model Code 2010 addresses issues concerning conservation strategies and tactics, conservation management, condition surveys, condition assessment, condition evaluation and decision-making, making interventions and the recording of information for through-life management.
Chapter 9 incorporates the overall philosophy adopted in the development of fib Model Code 2010, which introduces a new integrated life cycle perspective into the design of concrete structures. Accordingly, Chapter 9 provides a response to concepts introduced earlier within fib Model Code 2010 relating to the service life design process, which requires the structure and its component parts to be allocated to a condition control category at the time of design. Different condition control categories are defined depending on factors such as the importance of the structure, its function, design service life, impact on third parties, environmental conditions, ease of maintenance and cost. The condition control levels and inspection regimes are defined in conjunction with these requirements. A through-life management process, outlined in Chapter 9, provides feedback for service life design and allows the associated theoretical model employed to be updated, in turn facilitating the assessment of compliance with the original design objectives.
An example of concrete structure conservation according to the fib Model Code 2010 concept is also presented.

Fibre-reinforced polymers (FRP) in the form of externally bonded reinforcement have been used successfully in the rehabilitation of concrete structures. Although considerable data has been produced on the performance of strengthened RC structures, the reliability of strengthened structures can be significantly reduced due to the variability in the FRP properties, especially when the wet layup technique is used. In addition to this, structural engineers are concerned about the durability of FRP-strengthened structures under extreme loading conditions. Nonetheless, knowledge of the behaviour of strengthened elements under fatigue loading may be important to raise confidence in the strengthening systems. This paper presents the results of an experimental programme developed to investigate the behaviour up to failure of RC beams strengthened with high-performance carbon and aramid fibre sheets and subjected to static and cyclic loadings in terms of ultimate loads, deflections, cracking behaviour, failure modes and fatigue life by means of loading, crack width and deflection monitoring. Experimental data on fatigue life were used to validate analytical models developed for strengthened and unstrengthened beams.

Im Mittelpunkt des Beitrages stehen nichtlineare Materialmodelle, die das globale Tragverhalten von Stahlverbundstrukturen wiedergeben. Ein Vorteil ist dabei, daß die Anzahl der Materialparameter gering gehalten werden kann und diese zudem über Normen wie DIN 18800, EC2 und EC4 oder anhand des CEB-FIP Model Code bestimmt werden können. Die Materialmodelle der drei Querschnittskomponenten Stahlbeton, Stahl und der Verbundmittel berücksichtigen ein ver-/entfestigendes Materialverhalten. Eine erste Phase sieht die Anwendung dieser Materialmodelle innerhalb zweidimensionaler Finiter Elemente vor; typische Stahlverbundträger, bestehend aus Stahlbetonplatten als Gurtplatte auf Stahlträgern, verbunden durch Kopfbolzendübel, werden simuliert. In weiterführenden Arbeiten können diese Materialmodelle problemlos auf dreidimensionale Finite Elemente übertragen werden.

Strain penetration of the longitudinal reinforcement in reinforced concrete (RC) members at the joints and/or footings results in fixed-end rotations at the member ends. Several experimental studies have shown that fixed-end rotations caused by strain penetration contribute significantly (up to 50 %) to the total displacement capacity of RC members. Hence, accurate determination of these fixed-end rotations at yielding and ultimate limit states is of primary importance when defining the structural response of RC members. The purpose of this study is to present the theoretical background to and the assumptions made for the most common relationships found in the literature for determining strain penetration-induced fixed-end rotations at yielding and ultimate. Furthermore, new simple relationships are proposed on the basis of realistic and mechanically based assumptions. Comparisons between the existing and proposed relationships demonstrate the limitations of the former. Finally, the proposed relationships are calibrated against experimental measurements of RC column specimens subjected to cyclic loading with recorded fixed-end rotations due to strain penetration in the adjacent joints and/or footings.

This paper presents a five-spring model capable of predicting the complete pre- and post-peak shear behaviour of deep beams. The model stems from a two-parameter kinematic theory (2PKT) for the shear strength and displacement capacity of deep beams under single curvature. Four of the springs of the model represent the shear resistance mechanisms of the beam, while the fifth spring models the flexural behaviour. The model predicts not only the load-displacement response, but also the deformation patterns of the beam and how these patterns change with increasing load. Validation studies are performed by using 28 tests from the literature, showing excellent results. The model is used to interpret the tests and to draw conclusions about the behaviour of deep beams. It is shown that shear strength variations of up to 60 % between nominally identical specimens can be caused by variations in the path of the critical shear cracks. It is also demonstrated that loss of bond of large reinforcing bars increases the shear capacity of deep beams. Finally, the five-spring model is shown to predict the post-peak shear behaviour effectively, which is important for the analysis of structures under extreme loading.

In order to address how new knowledge influences design expressions, design codes have in most cases become significantly more complex over the last decades. However, this tendency is leading to codes that are too complicated for preliminary design but still not sufficiently accurate for assessing existing structures (where even more realistic models of behaviour are sometimes required). An alternative code strategy is that proposed by codes based on a levels-of-approximation (LoA) approach. This approach is based on the use of theories based on physical parameters where the hypotheses for their application can be refined as the accuracy required increases. The approach proposes adopting safe hypotheses during the first stages of design, leading to relatively quick and simple analyses. In cases where such a degree of accuracy is not sufficient (e.g. design of complex structures, assessment of existing structures, significant potential economic savings), the hypotheses can be refined in a number of steps, leading to better estimates of the behaviour and strength of members. This approach, recently adopted in the first complete draft of Model Code 2010 for a number of design issues, is discussed within this paper with reference to punching shear provisions.

This paper outlines the theoretical background to the punching shear provisions implemented in the fib Model Code for Concrete Structures 2010 and presents a practical example of their application. The aim is to explain the mechanical model that forms the basis for the punching design equations, to justify the relevance of the provisions and to show their suitability for the design and assessment of structures.

Section 5.1 “Concrete” of the fib Model Code for Concrete Structures 2010 contains basic definitions and well-established constitutive relations for structural concrete. However, it also presents various new approaches and updated models compared with the earlier CEB-FIP Model Code 1990. This is particularly true for the strength, stress and strain characteristics of structural concrete, for creep and shrinkage and for sophisticated durability-related processes. The validity of the models has been extended to several types of concrete such as high strength concrete, self-compacting concrete and lightweight aggregate concrete. The durability-related models are either suitable for facilitating a full probabilistic service life design or for applying simpler code-type approaches.
This article provides a concise and selective overview of some of those models. Background information is summarized and there is a focus on improvements achieved by the updated models. In addition, some simple design aids are given to allow pre-design, for example

Im Zeitalter des Mobilfunks dienen viele turmartige Bauwerke als Antennenträger. Viele davon stellen die besonders dafür konzipierten Schleuderbetonmaste sowie alte, entsprechend nachgerüstete Schornsteine dar. Manche dieser Tragwerke weisen auffällige Vertikalrisse auf, deren Auswirkung auf die Tragfähigkeit begutachtet werden muß. Bei dieser Aufgabe muß in Betracht gezogen werden, daß das im Erfahrungsbereich liegende Rißverhalten normaler Betontragwerken nur bedingt auf Schleuderbetontragwerke übertragbar ist. In diesem Sinne befaßt sich der vorliegende Beitrag mit der Erforschung der Unterschiede im Rißverhalten der hochfesten Schleuderbetonmaste gegenüber dem der niederfesten Betonschornsteine. Die entsprechenden Untersuchungsergebnisse wurden mit der Rißnachweismethode der Normen DIN V 1056, DIN EN 13084 und CICIND Model Code gewonnen und stellen einen wichtigen Teil der Qualitätssicherung von turmartigen Tragwerken dar.

Sowohl die zentrische Matrixzugfestigkeit als auch die zentrischen Nachrisszugfestigkeiten von faserbewehrten Betonen werden in der Regel indirekt über Biegezugversuche ermittelt. Dazu muss eine in Versuchen ermittelte Kraft-Verformungs-Kurve in eine Zugspannungs-Dehnungs-Beziehung überführt werden. Für ultrahochfesten Faserbeton (UHPFRC) gibt es in Deutschland zurzeit mangels gültiger Normen und Richtlinien kein geregeltes derartiges Umrechnungsverfahren. Zur Untersuchung des Zugspannungs-Dehnungs-Verhaltens von UHPFRC wurden am iBMB, Fachgebiet Massivbau der TU Braunschweig gekerbte 3-Punkt-Biegezugversuche nach DIN EN 14651 durchgeführt. Unter Berücksichtigung der Versuchsergebnisse und in Anlehnung an den Model Code 2010, der normal- und hochfesten Faserbeton regelt, wurde ein Umrechnungsverfahren für UHPFRC entwickelt. Zur Validierung des Verfahrens wurde die Finite-Elemente-Methode (FEM) hinzugezogen.

Tensile Stress-Strain Relationship for UHPFRC based on notched 3-point-bending tests
Both the tensile strength and the residual tensile strengths of fibre reinforced concrete are generally determined indirectly by flexural tests. For this purpose, a force-deformation curve has to be converted into a tensile stress-strain curve. For ultra-high performance fibre reinforced concrete (UHPFRC) there is currently no regulated conversion procedure due to the lack of valid standards and guidelines in Germany. To analyse the tensile stress-strain behaviour of UHPFRC, notched 3-point-bending tests according DIN EN 14651 were carried out at iBMB, Division of Concrete Construction of the TU Braunschweig. A conversion procedure for UHPFRC based on the Model Code 2010, which regulates normal- and high-strength fibre reinforced concrete, was developed taking into account the test results. The finite element method (FEM) was applied for validation.

This article describes an experimental programme aimed at studying the effect of cover, ratio between diameter and effective reinforcement ratio (&phgr;/&rgr;s, ef) and the influence of stirrup spacing on the cracking behaviour of reinforced concrete elements. The experimental programme was conceived in order to contribute to the debate - fuelled by the publication in recent years of Eurocode 2 EN1992-1-1 and the revision of the Model Code under way when the tests were carried out (and now published as a finalized document) - regarding the influence of these parameters on cracking. Important theoretical aspects are discussed, including where the crack width is estimated by current code formulations and what relevance this may have on the correlation between crack opening and durability of RC structures, especially with regard to structures with large covers. The effect of stirrup spacing, a variable absent from current codes, is also discussed.

The effect of concrete grade on the bond between 12 mm diameter deformed steel bars and recycled aggregate concrete (RAC) has been investigated with the help of 45 pullout tests with concentric rebar placement for coarse recycled concrete aggregate (RCA) replacement levels of 25, 50, 75 and 100%. For all the three concrete grades, the measured bond-slip relationships indicate similar mechanisms of bond resistance in the RAC and the natural aggregate (NA) concrete. The most accurate and least conservative predictions of the measured bond strengths were obtained from the local bond-slip model in the fib Model Code for Concrete Structures 2010. Bond strength normalized to fc(3/4) resulted in an improved match with test data and increased with an increase in the RCA replacement levels and decreased with an increase in compressive strength. An attempt to explain this behaviour has been sought in terms of brittleness index, an analogous parameter from rock mechanics. An empirical bond stress versus slip relationship has been proposed for the 12 mm diameter bar and it is conservatively suggested that similar anchorage lengths for this bar in all three concrete grades can be adopted for the RAC and the NA concretes.

Eleven T-beams, reinforced with high strength steel, were tested to failure to investigate the effect of shear span to depth ratio, fibre ratio, fibre type, concrete strength and stirrup ratio on the shear behaviour, especially post-cracking shear strength and deformability, of ultra-high performance fibre reinforced concrete (UHPFRC) beams. Test results indicated that fibres were efficient not only in enhancing the post-cracking shear strength, but also in improving the post-cracking deformability of UHPFRC beams. In addition, fibres could bridge the cracks and help in redistributing and homogenizing the concrete stress beside the cracks, allowing more short fine diagonal shear cracks with small spacing to develop around the existing cracks. A moderate amount of stirrups can effectively restrain shear cracks and allow more parallel diagonal shear cracks to develop and propagate thoroughly within the shear span. The stiffness of the UHPFRC beams at ultimate state was about 50 % of initial beam stiffness, which was considerable in strength calculations and ductility analysis, especially in seismic performance evaluation. Lastly, the current shear provisions were evaluated using the experimental results.

Interface shear transfer between differently aged concretes is a topic that crops up frequently and in different situations in structural design. In the fib Model Code for Concrete Structures 2010 the fundamental basics of concrete-to-concrete load transfer are given in section 6.3 and the corresponding design rules in 7.3.3.6. The different potential mechanisms contributing to the shear resistance along the interface, i.e. adhesive bond, aggregate interlock, friction and dowel action, are thus combined and their relationship taken into account by interaction factors. This article summarizes the most important results from past and ongoing studies and presents the background to the theory forming the design basis of fib Model Code 2010, the “extended shear friction theory” (ESF).

When reinforcing bars are post-installed in holes drilled in cured concrete, adhesive mortars are used to create a bond between concrete and bars. Appropriate adhesives can develop higher bond strengths than standard ribbed bars cast into concrete. A detailed design concept for the anchorage length of reinforcing bars has been developed by taking into account splitting/spalling of the concrete and pullout. Pullout and splitting tests on reinforcing bars set in concrete were carried out with different adhesive mortars and with varying concrete strengths and concrete covers. When higher bond strengths than those recommended for cast-in reinforcement are taken into account, it is important to check deformations and crack widths at the serviceability limit state (SLS) separately. For this reason, structural tests on slabs and corbels were carried out. Moreover, pullout tests on post-installed reinforcing bars were performed in order to measure displacements at service load level.

The fib Model Code for Concrete Structures 2010 introduces a new design concept for punching shear based on critical shear crack theory. This paper presents and provides the background to the design provisions for punching shear according to fib Model Code 2010, Eurocode 2 and the corresponding German National Annex to Eurocode 2. The different punching shear design provisions are critically reviewed by means of parameter studies and a comparison of the calculated resistances and test results. The safety levels of the code provisions are verified and the influence of the different punching parameters on the calculated resistances is examined in detail.

Mit Model Code 2010 wurde ein neues Bemessungskonzept für den Durchstanznachweis vorgestellt, welches auf der Theorie der kritischen Schubrissbreite (Critical Shear Crack Theory) basiert. Im Rahmen dieses Beitrags werden die Durchstanzwiderstände nach Model Code 2010, Eurocode 2 und den Regelungen des deutschen Anhangs zu Eurocode 2 vorgestellt und um Hintergrundinformationen ergänzt. Anhand von Parameterrechnungen und der Nachrechnungen von Durchstanzversuchen erfolgt ein Vergleich der unterschiedlichen Bemessungskonzepte. Dabei werden Sicherheitsdefizite identifiziert und die Auswirkungen unterschiedlicher Einflussparameter auf die Durchstanztragfähigkeit von Flachdecken-Stützenknoten herausgearbeitet.

Comparison of punching shear design according to Model Code 2010 and Eurocode 2
Model Code 2010 introduces a new design concept for punching shear, which bases on the so-called Critical Shear Crack Theory. In this paper, the design provisions for punching shear according to Model Code 2010, Eurocode 2 and the corresponding German National Annex to Eurocode 2 are presented and background information is given. By means of parameter studies and a comparison of the calculated resistances to test result, the different punching shear design provisions are critically reviewed. The safety levels of the code provisions are verified and the influence of the different punching parameters on the calculated resistances is examined in detail.

High-performance concretes such as high-strength concrete (HSC) or lightweight aggregate concrete (LWAC) are generally used to reduce member sizes and self-weight, and to optimize the construction of reinforced concrete structures. The bond between the aggregate particles and the cement paste can be strong enough in HSC and LWAC to cause the aggregate to fracture at cracks, which in turn reduces the shear stress that can be transferred across cracks by means of aggregate interlock. Relatively smooth cracks can also develop in self-compacting concrete due to the low coarse aggregate content. The contribution of aggregate interlock to the shear strength of RC beams is uncertain and depends on parameters such as the amount of shear reinforcement or the contribution of arching action for loads applied close to the support. Existing tests on slender RC beams without shear reinforcement have shown that shear strength is reduced by aggregate fracture. However, there is a lack of similar test data for members with stirrups and for members with varying shear span/effective depth ratios. This paper reviews the findings and contributions in this area from the experimental and analytical research of the author's PhD thesis, which was awarded the fib Achievement Award for Young Engineers in 2011.

Considering the depletion of resources and energy and the risks of climate change on a global scale, a thoughtless increase in the use of resources and energy in the construction sector is obviously unacceptable. The sector has until now constructed a system of technology focused on safety and comfort, with priority given to economic and social benefits. Such demands remain extremely important; however, in the future we ought to give additional consideration to the depletion of resources, energy consumption and other, ensuing environmental issues. This means that the sector needs to incorporate sustainability - including the environmental, economic and social aspects - into its systems of design and technology. The fib decided to incorporate a “concrete sustainability” concept in its new fib Model Code for Concrete Structures 2010. This paper explains sustainability as expressed in this code together with the background to it. In addition, the essence of sustainability with respect to future Model Codes is discussed.

Concrete has become the most used material on Earth over the 200 years following the invention of modern cement. The design concept has undergone a transition from the allowable-stress design method, limit-state design method, to the performance-based design method, in response to the evolution of materials, sophistication of experimental facilities, and advancement of computation skills. From the issues on resources and energy depletion, global warming, and resilience etc., it is necessary to create a new design framework taking into consideration the required performance beyond the conventional concept, in order to construct infrastructure and buildings in a more rational way. In other words, we should construct a design system that sets the continued existence of the diverse and rich global environment as its most important criterion of value. In this paper, we review the design and technology system developed in the past and discuss it based on the above-mentioned new viewpoint, while constructing and presenting a new design system for concrete structures, focusing mainly on the concept of sustainability, which is regarded as the most important factor in achieving conservation of Earth's rich resources as well as sound socio-economic activities of humankind in the future, and we examine its feasibility.

All researchers who have tested the shear capacity of RC members without stirrups have observed a large scatter in the results.
The objective of this paper is to conduct an overview study of the causes of the great shear failure scatter of RC beams without stirrups. Thirteen groups of shear tests on comparable experiments, extracted from the ACI-DAfStb evaluation database, are considered. The amount of data available is increased numerically. To this end, based on Eurocode 2 equations for shear resistance and shrinkage strain, a full probabilistic model is defined according to the JCSS Probabilistic Model Code. A multivariate statistical evaluation of outcomes is then performed.
The investigation highlights the fact that both the tensile strength of concrete and high shrinkage values may be usefully considered for more in-depth studies of the phenomenon, whereas geometrical properties and concrete compressive strength seem to be less important or can even be neglected.